This invention relates to desolventizers for recovering residual solvent from solids, such as the flakes or meals of seeds which have had their oil and fat previously extracted by solvent extraction. The invention can also be applied to any solid on which the operation of solvent extraction has been performed. More particularly, this invention relates to an improved deck for use in a desolventizer, or the like.
Solvent-laden meals are obtained in the process of extracting oils and fats from seed materials, such as soybean, grape, sunflower seed, nut material, and the like. Solvents or extractants typically used for this purpose are, for example, benzene, propane, butane, pentane, hexane, or mixed solvents, such as mixtures of the above-mentioned hydrocarbons with alcohols, ketones, or generally polar solvents. The miscella formed in the extraction of the above-mentioned raw materials is separated from the residue (meal) and forwarded for further processing such as purification or distillation, or the like.
The residue of the extraction process, that is, the mixture of meal, solvent, and water, is treated in a desolventizing apparatus for the purposes of recovering the solvent and obtaining a low-solvent or solvent-free meal. Such meals may contain as much as 35% by weight of solvent prior to the desolventizing process.
Processes and apparatuses suitable for desolventizing solvent extracted solids have long been known. In these devices, solvent is typically evaporated from the meal by steam or steam-containing fluids with the solvent returned to a recovery apparatus such as a fractional distillation unit or the like in a mostly continuous operation. For example, U.S. Pat. No. 2,806,297 to Hutchins, and U.S. Pat. No. 4,619,053 to Schumacher, disclose apparatuses of the countercurrent steam injection type for separating solvent from solvent laden solids in a continuous process.
Typically, such apparatuses consist of a plurality of chambers or compartments arranged in a vertical configuration. Solvent laden solids may be introduced into the uppermost chamber where they pass downwardly from an upstream to a downstream direction from chamber to chamber. The treated meal may be removed from the lowermost chamber. Live steam, or steam-containing vapors may be introduced below the deck of the lowermost chamber. The steam rises in countercurrent fashion through the solvent laden solids, with the solvent vaporizing. A mixture of steam and evaporated solvent is then channeled from the uppermost chamber to conventional recovery equipment.
The decks between chambers may include discharge passages or chutes for the downward movement of meal. The decks may also include bores or perforations to permit the upward movement of steam and evaporated solvent. The apparatus may include a centrally disposed vertical shaft running through the chambers with stirrers mounted on the shaft for rotation within each chamber. The stirrers may pass just above the top surface of the decks so as to keep the meal in a constant state of agitation.
One or more of the upper chambers may be used for pre-desolventizing. For this purpose, one or more of the upper decks may be constructed of double-walled imperforate steel, and have a central aperture surrounding the vertical shaft to permit the upward passage of steam and evaporated solvent. Steam may be admitted into the space between the double walls so as to provide indirect heating of the solvent laden solids.
One or more of the lower chambers may be used for stripping or toasting the meal after most of the solvent has been evaporated from the meal. For this purpose the decks may consist of a double wall construction having bores or perforations therethrough. The double wall construction permits steam to be admitted into the deck thereby heating the deck and inhibiting the condensation of steam thereon. The bores or perforations permit steam that is admitted below the deck to travel upwardly through the meal without substantially condensing in the meal.
In the chamber where most of the desolventizing may take place, the steam or steam-containing vapors admitted from below the deck rise through the solvent laden solids and may condense therein, thereby evaporating the solvent contained in the meal via the latent heat of evaporation of the condensing steam.
In some cases, the combination of liquid water and the high protein content in the meal can cause agglomeration of the meal, producing what are known in the art as "water balls". These water balls may be golf-ball-sized clumps of meal and water that are substantially impervious to the steam. As a consequence, solvent trapped in the water balls may not be removed by evaporation. This can lead to localized areas within the processed meal that have higher than desired concentrations of solvent.
In some of the prior art designs, undesirably high pressure drops are experienced as measured from downstream to adjacent upstream treatment zone. Often, this problem results from clogging of the perforations or bores that are provided in the decks that, ideally, allow steam communication from a downstream treatment zone to its adjacent, upstream treatment zone.
Additionally, when such clogging occurs, the steam primarily travels upwardly through the meal discharge chutes that are disposed in the deck surface to provide for passage of the meal through the deck from an upstream to downstream zone. When this happens, the meal can be impeded from making proper passage, thus increasing the chance that it will not be evenly distributed around the deck.
Typical designs provide deck bores or perforations in an amount of about 1-3% open deck space. These designs may be of some value in increasing steam velocity through the holes in the deck to inhibit the plugging or clogging problems, but the concentration of steam pressure at these few, discrete openings increases the likelihood of steam plume formations that drive deeply into or even divide the solvent laden meal into a plurality of segregated, clumpy masses resulting in non uniform solvent evaporation and possibly in the "water-ball" phenomenon referred to above.
Accordingly, there is a need in the art for the provision of a deck structure that will promote more uniform steam pressure distribution from treatment zone to treatment zone and help solve the prior art problems referred to above.